Adds SVCB parameter key 7 (dohpath)

[okanaganrusty]

PR to add support for dohpath in the SVCB request.

Supporting issue #501

https://datatracker.ietf.org/doc/html/draft-schwartz-svcb-dns-00#section-5.1

[wcawijngaards]

Thank you for the code! This looks like it implements the dohpath option for SVCB and HTTPS fine. The document is still a draft, and because we do not want to create a too-early deployment issue, we do not want to ship this at this early stage. It would be best if we waited for discussion at the IETF to complete.

Meanwhile, for experimentation there can be code. The code from the diff of this pullrequest can perhaps help people that want it. Deploy by applying the patch at https://github.com/NLnetLabs/unbound/pull/547.diff and then dohpath should work. Another way, but perhaps later, is to add compile time switches that default to off, that turn on this new feature.

[jp-coding]

Adressed by #739

[gthess]

This seems to be addressed by #739 which is now merged; closing this.